A digital-to-analog converter (Digital-to-Analog Converter, DAC for short) converts a digital signal into an analog signal which may be sensed by people, and usually converts a digital code into a corresponding voltage signal through weighted summation. The DAC may be applied to, but not limited to, applications of video and image processing and the wireless field. The foregoing applications usually demand a high-speed and high-precision DAC, and the improvement of the speed and resolution usually increases design cost and power consumption. Therefore, it is particularly important to improve the performance of the DAC from a system and method level.
FIG. 1 is a schematic diagram of a typical oversampling SIGMA-DELTA DAC. As shown in FIG. 1, a SIGMA-DELTA modulator 102 converters an M-bit input signal 150 into an N-bit modulation signal through modulation, where M>N, and then a decoder 104 converts the N-bit modulation signal into a 2^N-bit thermometer code, a scrambler 106 dynamically matches the thermometer code, and finally a DAC circuit 108 converts the dynamically matched code into an analog signal 152. When quantization bits of a quantizer in the modulator 102 increase from N to N+1 in number, output lines of the decoder 104 increase from 2^N to 2^(N+1) in number, and dynamic algorithm units in the scrambler 106 and DAC units in the DAC circuit 108 also double in number. For example, when N increases from 6 to 7, the output lines of the decoder 104 increase from 64 to 128 in number, and the dynamic algorithm units and the DAC units also increase from 64 to 128 in number.
In the prior art, there is a kind of DAC, which is equivalent to a cascaded modulator in structure, where the input of a latter stage is quantization noise output by a former stage, first stage quantization noise is finally offset through summation of an analog part, and finally, only high-order modulated quantization noise is left. In the structure, signals only exist in a first stage modulator, a latter stage modulator only includes noise, and noise in a band may be moved out of the band by increasing the order of the modulator, so noise out of the band is higher. If in-band noise and out-of-band noise are reduced on the whole, the main method is to adopt a quantizer with more quantization bits in the modulator.
However, an increase of the quantization bits of the quantizer brings non-ideal factors such as a static mismatching error of a current source and a non-ideal characteristic of a switch, and the factors may result in generation of a harmonic wave and deteriorate quality of an output signal. In addition, the increase of the quantization bits of the quantizer needs to correspondingly increase the number of the dynamic algorithm units and the DAC units, causing an increase to the power consumption and an overall DAC area.